Anodic treatment of magnesium



Patented July 2, 1946' PATENT oFF ca mm anomc mum! or moussmu Robert w. Hazard, Remington, us.

No mam. Application November 21, a. Serial No. 113,:

(Granted m a, m of March 3, ms, as" amended April as, lmi m o. a. m)

This invention relatesto. a method of and a bath for forming corrosion-resisting coatings on magnesium and the alloys thereof bypassing current through the article to be coated while it is functioning as the abode in a bath having suitable properties.

It is an object ofthis invention to provide a bath for the formation of protective films on magnesium and its alloys over a wider current density range than in baths heretofore A further object is to provide a bath that will supply ionsto combine with the magnesiuminthe formation of such films. Still another object is to proviiie a bath wherein the pH is maintained at values as will result in the formation of coatings having superior mechanical and corrosion-resist-' ing characteristics. An additional object, is to provide a' plurality of baths that may be used in the formation of satisfactory films on magnesium and its alloys. a

The invention'disciosed and claimed herein is an improvement on and extension of the subject matter of my copending application, Serial No. 13,911, filed March 30, 1,935. it It has been known for some years that articles of aluminum could be protected against corrosion by a film formed thereon when the article was usedastheanode inabathhavingasuitable composition, but the substances used for anodiz- 80 ingthis metal were found to be not .useful in forming protective coatings on magnesium. I

' have discovered that magnesium and its alloys may be satisfactorily treated in a bath that contains, broadly speaking, a substance that yields oxidizing ions and a substance that yields ions that will react with the magnesium and enter into the composition of the film formed thereon. The coating applied by the process herein disclosed varies in color from a light dirty green to a black. x

40 The oxidizing ion may be derived from oxalic acid, acetic acid, nitrates, chlorates, permanganates or dichromates, while the reacting ions may be derived from sulphates, phosphates, borates, fluorides, or molybdates. I The salts used as sources of the oxidizing ions are preferably those of sodium, potassium, or ammonium. Satisfactory films have been produced with the following combinations: nitrate and sulphate; oxalic acid and phosphate; permanganate and phosphate; chlorate and sulphate; chlorate and phosphate;

molybdate and phosphate: nitrate and phosphate;

. nitrate, borate andphosphate; dichromate, borate and molybdate; permanganate and sulphate; chromate, or oxalic acid or acetic acid and sul- I T The method of practicing my invention is as follows:

The article to be coated is thoroughly cleaned, a clean surfacebeingessential to the formation of good Electrolytic cleaning is very satisfactoryalthou'gh other known methods of accom- 'plishing result may be used instead. After and washing, the article is made the anode in a bath having the constituents above mentioned in a concentration from one per cent up to-the limit of solubility of the respective saltsin the bath. The pH of the solution is preferably between 2 and 5, the temperature being between;

- 29 and centigrade and applied current rangingfroi'n 1'to'100 amperes per square foot. The I for treatment depends upon the strength of the solution, the current density, and the thicb. ness of film desired. As a specific example, when the currentgdensity is 5 to 10 amperes per square foot and the temperature of the bath is' 50 centi- D0 grade a very satisfactory film is formed in 45 to 80 minutes. As an example of the solution strength,

I have used 10 per cent dichromate and 2 per 'cent manganese sulphate, also 10 per cent.dichromate and 2 per cent chromium sulphate. The success- 26 ful use of sulphates of manganese and chromium, as stated, and of other metals, shows that the source of the reacting ions need not be a soluble salt of an alkali but may be any soluble salt containing the not include a substance detrimental to the metal being treated. While nitrates and other oxidizers may be used with good results I have found that the presence of chromate results in films that have higher corrosion inhibiting power than 35 when the chromate is absent. After the film has been formed the article is removed from the-solution, washed and dried.

There is evidence t the coating formed by the present method results from both anodi'c oxi- 40 dation and also deposition. A bath for practicing my invention herein-disclosed, therefore, con-v tains an oxidizing ion as a derivative of oxalic acid in the presence of a depositing ion or reacting agent such as sulphate. The pH of the bath is 4 preferably adjusted on the acid side to holdthe ingredients in solution.

The invention herein described and claimed may be used and/0r manufactured by or for the Government of the United States of America for so governmental purposes without the payment of any royalties thereon or therefor.

' I claim:

- 1. A bath for anodical coating magnesium or its alloys, comprising an aqueaus solution of 5' desired ions so long as the salt does 30 I 10 per cent water soluble dichromate and 2 per cent water soluble manganese sulphate, the bath having a pH range from 2 to 5.

2. A bath for anodically coating magnesium or its alloys, comprising an aqueous solution of 10 per cent water soluble clichromate and 2 per cent water soluble chromium sulphate, the bath having a pH range from 2 to 5.

'3. A method of forming protective coatings on magnesium and magnesium alloys in which magnesium predominates comprising passing an electric current having a current density of from 1 to 100 amperes per square foot through the metal while it is functioning as the anode in an aqueous solution containing a water soluble compound chromate and a water soluble sulphate, the pH of the solution being on the acid side and the substances being present in the solution in concentrations from 1 percent up to the limit of solubility of each substance.

4. A method of forming. protective coatings on magnesium and magnesium alloys in which magnesium predominates comprising passing an electric current having a current density of from 1 to 100 amperes per square foot through the metal while it is functioning as the anode in an aqueous solution containing a water soluble dichromate'and a water soluble sulphate, the pH of the solution being on the acid side and the substances being present in the solution in concentrations from 1 percent up to the limit of solubility of each substance.

5. A method of forming protective coatings on magnesium and magnesium alloys in which magnesium predominates comprising passing an electric current having a current density of from 1 to 100 amperes per square foot through the metal while it is functioning as the anode in an aqueous solution containing ammonium sulphate and a water soluble compound selected from group consisting of the chromates and dichromates, the pH of the solution being on the acid side and the said salts being present in the solution in concentrations from 1 per cent up to the limit of solubility.

6. A method of forming protective coatings on magnesium and magnesium alloys in which magnesium predominates comprising passing an electric current having a current density oi. from 1 to 100 amperes per square foot through the metal while it is functioning as the anode in an aqueous solution containing manganese sulphate and a water soluble compound selected from group consisting of the chromates and di chromates, the pH of the solution being on the acid side and the said salts being present in the solution in concentrations from 1 per cent up to the limit of solubility.

'7. A method of forming protective coatings on magnesium and magnesium alloys in which mag nesium predominates comprising passing an electric current having a current density of from 1 to 100 amperes per square foot through the metal while it is functioning as the anode in an aqueous solution containing chromium sulphate and a water soluble compound selected from group consisting of the chromates and dichromates, the pH of the solution being on the acid side and the substances being present in'the solution in concentrations from 1 percent up to the limit of solubility of the substance in the solution.

' ROBERT W. BUZZARD. 

